The invention relates to a method for manufacturing calendered paper. The invention also relates to an apparatus for manufacturing calendered paper.
After the paper has been dried, the surface structure of the web is made suitable by means of a mechanical treatment, calendering. There are several calendering methods, but it is common to all of them that the web is passed through one or several nips which are formed between two surfaces, typically between rotating roll surfaces. The purpose of the calendering is to improve the paper quality by pressing the paper into a fixed final thickness, and especially by smoothing its surface. As is well known, humidity improves the mouldability of the fibres contained in the paper in connection with the calendering. Because of this, the calendering is conducted in a fixed moisture content, xe2x80x9cthe target moisture of calenderingxe2x80x9d.
In the following, the calendering processes of certain SC-grades will be described.
Conventionally, SC-paper is manufactured in an off-line process. The paper is dried, xe2x80x9coverdriedxe2x80x9d, in the paper machine to a moisture of about 2 to 4%. Thereafter the paper is wetted to a calendering moisture of 8 to 12% before reeling up. It is typical for the process that the moisture has the time to equalize itself during the standing time of the reel in the z-direction (i.e., perpendicular to a surface of the paper) of the paper before calendering. In this context, the standing time refers to the time between the manufacturing time and the beginning of calendering.
In present on-line multi-roll calender applications of SC-grades the paper is dried in the drying section to the target moisture of calendering. Generally, it is typical for these processes that the paper is passed directly to the calender in the final target moisture of 8 to 12%, even 15%.
Normally, when paper is calendered, its surface is smoothed and glazed in the expense of the thickness of the paper. The controlled variable used in the calender is the linear load and/or nip pressure, which determines the quality of the surface of the paper as well as the final thickness. Especially in connection with printing papers with low grammage it has been impossible to implement such calendering in which the thickness of the paper would not be substantially reduced when aiming to the desired surface quality.
Up to the present, only theories have been presented as to how such a structure of the paper could be attained that the calendering work would be focused on the surface layers of the web, while the density in the central part of the web would remain nearly in its original state, i.e. the calendering would restore its xe2x80x9cbulkinessxe2x80x9d. Generally, the theories could be referred to by the term gradient calendering. It has not been established by anyone in a watertight manner that the manufacture of so-called gradient paper would be possible with printing papers.
The purpose of the invention is to present a real practical method for manufacturing calendered paper, the end product of which is a gradient calendered paper.
The method is based on heavy drying of the body moisture of paper, i.e. so-called overdrying, and on the wetting of this overdried paper before calendering in such a way that the wetting is conducted on a moving web, which is guided towards the calender. In this context, overdrying refers to the act of drying the paper to a lower moisture than the moisture normally allowed for the paper when the paper enters the calender nip, when it is taken into account that in the calendering, the moisture of the web is reduced further by over 2 percentage points, wherein the paper attained after calendering is dryer than the moisture of use of paper in normal conditions. The wetting is conducted at a suitable distance from the calender, so that when the web speed of the paper is taken into account, the moistening water has the time to be absorbed in the surface of the paper. On the other hand, the metering is conducted in such a way that the moistening water is absorbed only in the surface layer of the web, while the central layer remains substantially dry (in the moisture of an overdried web).
In the method, it is possible to use both on-line calendering and off-line calendering. In the former case, the web is overdried in the drying section of the paper machine and passed to the calender via wetting. In the latter case, the web is overdried in the drying section and reeled in a reel-up to form a reel, whereafter it is unwound to the calender via wetting.
The calender is advantageously a multi-nip calender which has several successive nips between the calender rolls, which exert a determined nip pressure on the web passed through the nip. To manufacture good printing paper grades, calenders with over 4 nips are typically used.
The apparatus includes a wetting device which is placed at a suitable distance from the calender. There may be wetting devices on both sides of the web to wet both sides of the web. Both wetting devices are located within a suitable distance from the calender, wherein the result is a symmetrically gradient calendered paper, in which the density of both surface layers is higher than the density of the central layer.